A magnetron sputtering device is used for depositing thin film layers on a substrate and utilizes a rotary cathode that includes a cylindrical hollow tube with a target material thereon. This target tube is rotated around a stationary magnet array suspended inside of the tube. The magnet array is directed at a substrate in a vacuum chamber and holds processing plasma in a desired location for coating the target material on the substrate. A coolant such as water typically flows inside the target tube for cooling during the sputtering process.
Some target materials need more cooling than others in order to keep them from being damaged. As target materials and rotary targets are very expensive, it is important to maintain their useful life by preventing damage where possible to avoid replacement costs.
It is desirable to put as much coolant as possible back into the cooling system after the cathode has been drained. Coolant, even water, is expensive, and what is lost needs to be replaced. Cooling water is typically de-ionized, and treated with expensive anti-bacterial and anti-corrosion chemicals. The target tubes can be very long, such as up to about 4 meters in length. There can be many of these tubes in one sputtering system (e.g., over 50 in some cases), which amounts to a significant volume of coolant.
In a conventional water fill method for a rotary cathode, water is pumped into the inlet of a rotary feedthru passage on one end of the sputtering device and returned to an outlet at the same end. This is typically done by routing the water thru a central tube that supports the magnet array. The water exits the central tube thru holes along an end portion of the central tube and partially fills the target tube, leaving an air pocket at the top of the target tube.
There are several shortcomings in this conventional water fill method. In a horizontal application, if the magnet array is directed upward for instance (‘sputter up’), the magnet array is not totally in the cooling water but is partially in the air pocket. The process plasma, which generates the target heating, is then on the air pocket side of the target. In this case, the power to run the process must be reduced to avoid target damage, resulting in slower processing and thus reduced productivity. Additionally, in the sputter up scenario, if for some reason the target unintentionally stopped rotating, and this was not immediately detected, the target would be quickly destroyed. Not only would the target material be damaged, but the target tube would be bowed from one sided heating.
In addition, the conventional method does not allow for vertical applications. In this case, the air bubble goes to the top of the target tube. Therefore, there is no cooling available for the entire end of the target in that region, resulting in target damage, magnet assembly damage, as well as a potential vacuum leak because of heat damage to the end seal.
When cooling water is to be removed from the target tube in the typical method after the sputtering process, compressed air is used to ‘blow down’ the target tube. The water flow is shut off, and thru a valve mechanism, compressed air is pushed thru the same path in the rotary feedthru passage and the target tube to remove the water.
It is necessary to remove water from the target tube in order to avoid water spills into the vacuum chamber. Some cathode designs are such that the target tube is removed in the vacuum chamber and water is spilled out into the chamber. This results in added clean-up time, as well as addition time needed to pump all hidden water out of the vacuum chamber, so that the sputtering process can be started up again. This can cost many hours in lost production time, thus, water spills are very expensive.
It also necessary to remove water from the target tube in order to avoid water spills outside of the vacuum chamber. Some cathode designs are such that the target tube is removed from the cathode off line, outside of the vacuum chamber. Often, there are many gallons of water in these assemblies and when disassembled, this water pours out onto the cathode equipment and floor or, if possible, into a specialized water catching tool. This tool is often ineffective in catching all of the water as it gushes out onto the equipment. Dealing with this water and its clean up are inconvenient, time consuming, and raise various safety issues.
As the conventional method of coolant routing makes total water removal in the target tube impossible, all of the above problems occur frequently.